Small Spherical Tokamaks and Their Potential Role in Development of Fusion Power

Small Spherical Tokamaks and their potential role in development of fusion power

Dr David Kingham, Nuclear Futures, 26 March 2013

Plasma in START tokamak, Courtesy Euratom/CCFE Fusion Association

March 2013 © Tokamak Solutions UK Ltd 2013 1 Introduction

• Tokamak Solutions UK Ltd was established “to make fusion useful quickly” by developing spherical tokamaks and powerful fusion neutron sources • Based at Culham, the world leading centre for fusion (JET) with unique capabilities in compact “Spherical Tokamaks” (MAST, START) • The mainstream world fusion programme is focussed on a long term goal of developing fusion as an energy source using large tokamaks • Recent breakthroughs in manufacturing of high temperature superconductors mean we can now see a possible way to achieve a high magnetic field in a small spherical tokamak • Such devices could help to accelerate the development of fusion energy A record breaking plasma in the START spherical tokamak. The plasma diameter is < 1.5m.

March 2013 © Tokamak Solutions UK Ltd 2013 2 Some facts about fusion power

• There are two main candidates: magnetic and inertial confinement • Magnetic confinement is the most promising: there are several variants including Stellarators, Spheromaks and Tokamaks. • Tokamaks have by far the best performance: JET at Culham produced 16MW fusion power in a short pulse in 1998. • The fuel (deuterium and lithium) is cheap and plentiful • JET is a large conventional tokamak; ITER is even larger. ITER tokamak: 35m high; €15bn cost; due to demonstrate fusion power in 2027

March 2013 © Tokamak Solutions UK Ltd 2013 3 Why are spherical tokamaks so exciting?

• Spherical Tokamaks (STs) can be small and straightforward to build • STs are close enough to the mainstream to be confident of the science, but distinctive enough to allow development of proprietary technology • STs use a magnetic field much more effectively than a conventional tokamak • If only it was possible to achieve a high magnetic field in a small spherical tokamak…

March 2013 © Tokamak Solutions UK Ltd 2013 4 High Temperature Superconductors (HTS)

Recent developments in ‘High Temperature’ superconductors could have far-reaching implications

At first, these were just thought to be a more convenient form of LTS in that they give similar performance but at around 77K (liquid nitrogen) rather than 4K (liquid helium) temperatures.

March 2013 © Tokamak Solutions UK Ltd 2013 5 Properties of HTS…

HTS has a remarkably high critical current at high field and low temperature

Example: in a field of 5T, HTS at 20K can pass 100 times as much current as at 77K

March 2013 © Tokamak Solutions UK Ltd 2013 6 Using HTS magnets on a tokamak

We have performed the first test of an HTS magnet on a tokamak, by fitting two HTS poloidal field coils on the GOLEM tokamak at Prague.

Upper PF coil cryostat winding the coil filling with liquid nitrogen

We are now running the ST25, a small test tokamak, and plan (in conjunction with Oxford Instruments) to test out a full magnet system wound from HTS. We are also designing high field HTS-based Spherical Tokamaks

March 2013 © Tokamak Solutions UK Ltd 2013 7 ST25 (copper coils)

RF waveguide (EBW plasma current drive experiments)

8 limbs, each 14 turns; the high inductance together with the high capacitance of our ‘supercapacitors’ (8F) provides pulses > 2s

Development programme includes: • Conversion to HTS (High Temperature Superconducting) magnets • Studies of RF start-up and ramp-up via EBW (Electron Bernstein Waves) 2-s long RF pulse

This 6-turn cryostat has the returns at large enough radius to minimise field ripple and provide aperture to insert and assemble vac vessel

Approx cost of HTS tape: $100k

• Small Spherical Tokamaks can be built relatively easily and cheaply • They are exciting because they use a magnetic field efficiently • It now looks possible to design and build high field spherical tokamaks • Such devices could help to accelerate the development of fusion energy

Courtesy Euratom /CCFE Fusion Association